CO2 has stayed down there for over a million years, but it dissolves slowly.

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Reducing our emissions of carbon dioxide quickly enough to minimize the effects of climate change may require more than just phasing out the use of fossil fuels. During the phase-out, we may need to keep the CO2 we're emitting from reaching the atmosphere—a process called carbon capture and sequestration. The biggest obstacle preventing us from using CCS is the lack of economic motivation to do it. But that doesn't mean it's free from technological constraints and scientific unknowns.

One unknown relates to exactly what will happen to the CO2 we pump deep underground. As a free gas, CO2 would obviously be buoyant, fueling concerns about leakage. But CO2 dissolves into the briny water found in saline aquifers at these depths. Once the gas dissolves, the result is actually more dense than the brine, meaning it will settle downward. With time, much of that dissolved CO2 may precipitate as carbonate minerals.

But how quickly does any of this happen? Having answers will be key to understanding how well we really sequester the carbon.

They're difficult questions to answer accurately, because it really requires working with the complex environment of those deep, saline aquifers—not exactly the most convenient laboratory. A new study led by Kiran Sathaye of the University of Texas at Austin takes advantage of a natural CO2 reservoir to learn more.

Bravo Dome

That natural reservoir is New Mexico’s Bravo Dome—a sandstone layer trapped between impermeable rocks above and below. Bravo Dome holds nearly pure carbon dioxide gas, along with briney water. The CO­2 got there as a result of deep volcanic activity in the region, which produced gas that traveled up into the sandstone through fractures in the underlying bedrock. The sandstone is not perfectly horizontal, but it warps into a slight dome that collected the buoyant CO2.

Bravo Dome was tapped for its CO2 starting in the early 1980s, and it's been studied in exceptional detail. Lots of information has been gathered over the years, from the distribution of gaseous and dissolved CO2, to the character of the sandstone, to the faults that cut across it. No CO2 leakage has ever been detected.

It was thought that the volcanic SodaStream that filled Bravo Dome did so roughly 10,000 years or ago, but this wasn’t a solid estimate. For this latest study, the researchers took advantage of a technique often used to find out how long ago a rock rose to the surface (one we’ve described before). The technique involves analyzing the number of helium-4 atoms inside crystals of the mineral apatite. That helium-4 is the product of the decay of radioactive isotopes of uranium and thorium. If the apatite crystal is cooler than about 75°C, the helium-4 atoms will be stuck inside. At warmer temperatures, the helium-4 can escape.

Hot rocks

So by measuring the amount of helium-4 in the crystal, and given the rate that the uranium and thorium decay, you can tell how long it has been since the crystal cooled below 75°C. Since the CO2 at Bravo Dome is volcanic, the bits of apatite in the sandstone should have been heated up when that gas came through, clearing out any accumulated helium-4 and starting a fresh clock.

The researchers collected apatite samples from a couple sandstone cores—some taken near the fracture zone believed to have transmitted the rising volcanic gas, and others at a site 17 kilometers away. The far samples came back with ages of 12 to 17 million years, but the apatites near the fractures recorded a cooling age of 1.2 to 1.5 million years. If we view that as the time the rock was introduced to the hot volcanic gas, this indicates that the CO2 has been down there much longer than previously thought.

If you know how long the CO2 has been down there, and you know how much has dissolved into the brine, you can come up with an estimate of the dissolution rate—one that applies to representative, real-world conditions. From the plethora of existing measurements, the researchers calculate that about 22 percent of the CO2 has dissolved. The rest (minus the portion that humans have extracted) remains as a gas.

There are, however, a couple different processes driving that dissolution. While CO2 was being actively injected, the mixing drove rapid dissolution. After things settle down, and CO2 formed a pocket against the roof of the dome, dissolution slowed. Since CO2 dissolved into brine increases its density, the brine at the interface with the CO2 can sink as it becomes more dense, driving convection that brings a constant supply of “fresh” brine to interact with the CO2.

This convection process is one that researchers have wanted to get a better handle on.

Mixing it up

Based on measurements within portions of the brine, the researchers estimate that about 40 percent of the CO2 in the brine dissolved during the injection of the volcanic gas, with the rest dissolving over time—aided by the brine convection. That puts the average rate of dissolution over time at just 0.1 grams of CO2 per year for each square meter of contact between CO2 and brine.

That’s pretty low, and it's likely the result of the fact that water cannot flow through this sandstone very quickly. In the less-restrictive sandstone of the Sleipner reservoir beneath the North Sea, for example, where CO2 injection has been a part of natural gas production activities, dissolution rates of 20 kilograms per year have been estimated. Unfortunately, the sandstone at Bravo Dome is probably much more representative of the rocks we’d be using to store CO2 in the US.

When carbon dioxide storage projects are considered, a secure storage timeframe of 10,000 years is considered useful. What this study implies is that most CO2 dissolution into brine will take place during injection. Not much else will change in the 10,000 year timeframe due to convection.

However, learning how long the CO2 at Brave Dome has been trapped down there illustrates that, like natural gas reservoirs, these geologic storage lockers can effectively hold onto their contents for the long haul.